An unanswered question, fundamental to understanding the function of the retina, is how the absorption of light by rhodopsin is translated into a nerve impulse. This process would seem to depend on an interaction of internal structures of the rod outer segment with the plasma membrane. In other tissues, cyclic nucleotides serve to carry information from one part of the cell to another. Our preliminary findings that cyclase, phosphodiesterase and cyclic nucleotide-dependent protein kinase are present in purified rod outer segments strongly suggest that cyclic nucleotides participate in visual function. In this project we will characterize the enzymes involved in cyclic nucleotide metabolism in the rod outer segment, with emphasis on the proposed site of their action: the cyclic nucleotide-dependent protein kinase. The endogenous protein substrate for this enzyme will be purified and attempts will be made to characterize it. Determination of the function of this substrate protein should give important clues as to the mechanism of cyclic nucleotide action in the retina. By the use of radioactive tracers and immunological techniques, the effect of light on the activity of this protein kinase will be determined in the intact animal. Cyclic nucleotide levels in the outer segment region will be measured in parallel experiments. These results will be compared to light effects on enzyme activities in isolated rod outer segments and on purified enzymes. These studies should increase our understanding of visual function at the molecular level and suggest further experiments of a fundamental nature as well as potential ways in which to deal with retinal dysfunction and degeneration.